Various techniques are known for dispersing hydrophobic photographically useful compounds such as photographic couplers into photographic element layer coating compositions comprising hydrophilic colloids.
Photographic dye forming couplers, as well as other hydrophobic photographically useful compounds, are typically incorporated into a hydrophilic colloid layer of a photographic element by first forming an aqueous dispersion of the couplers and then mixing such dispersion with the layer coating solution. An organic solvent is typically used to dissolve the coupler, and the resulting organic solution is then dispersed in an aqueous medium to form the aqueous dispersion.
The organic phase of these dispersions frequently includes high boiling or permanent organic solvents, either alone or with low boiling or water miscible solvents which are removed after dispersion formation. Permanent high boiling solvents have a boiling point sufficiently high, generally above 150.degree. C. at atmospheric pressure, such that they are not evaporated under normal dispersion making and photographic layer coating procedures. Permanent high boiling coupler solvents are primarily used in the conventional "oil-protection" dispersion method whereby the organic solvent remains in the dispersion, and thereby is incorporated into the emulsion layer coating solution and ultimately into the photographic element.
The conventional "oil in water" dispersion method for incorporating hydrophobic couplers is described, e.g., in U.S. Pat. No. 2,322,027 by Jelly and Vittum. In such conventional process, the coupler is dissolved in a high boiling water immiscible solvent, mixed with aqueous gelatin, and dispersed using a colloid mill or homogenizer. The presence of the high boiling solvent provides a stable environment for the hydrophobic coupler, as well as generally increasing the reactivity of the coupler upon photographic processing.
U.S. Pat. No. 2,801,170 of Vittum et al. discloses preparing separate dispersions of a coupler and a high boiling point solvent and mixing the two dispersions with a silver halide emulsion. U.S. Pat. No. 2,787,544 of Godowsky et al. discloses a method of making mixed packet photographic systems. A dispersion of high boiling point solvent is mixed with a dispersion of coupler. Both these processes help prevent crystallization of the coupler prior to layer coating by keeping the solvent and the coupler separate until just prior to coating, while providing solvent in the coated layer to enhance coupler reactivity in the photographic element.
While the presence of high boiling solvents in certain coupler dispersions in photographic elements is frequently desirable to provide, e.g., adequate coupler reactivity upon photographic processing, lower thermal yellowing, modified dye hues, enhanced dye dark stability, and reduced crystallization, there are also certain advantages which may result from the substantial elimination of high boiling permanent solvent from a photographic element imaging layer. U.S. Pat. No. 5,173,398, e.g., discloses photographic elements with coupler-containing layers having substantially no high boiling solvent. Alternative methods for delivering hydrophobic materials, such as color couplers, to aqueous based photographic compositions without using high boiling solvents are well known in the art. Such techniques involve, e.g., deposition of the couplers from basic aqueous solutions or volatile low boiling or water miscible organic auxiliary solvents followed by removal of the auxiliary solvent by evaporation or washing, mechanical dispersion methods wherein solid compounds are directly dispersed in an aqueous composition, and latex loading methods wherein, e.g., a coupler is loaded into a latex polymer by mixing a solution of the coupler in a low boiling or water miscible organic solvent with an aqueous suspension of the latex followed by removal of the solvent. Minimizing the amount of permanent coupler solvent coated in a photographic element is useful, e.g., for reducing the coated thickness of photographic layers. Reductions in coupler solvent level also afford concomitant reductions in gelatin level which leads to further reductions in coated dry thickness. Layer thinning is advantageous for reasons such as improved image sharpness due to reduced light scattering during exposure and increased developability due to shorter diffusion paths through the multilayer structure. This increase in developability can lead to lower silver and/or coupler coated levels, hence lower materials cost.
The use of substituted bisphenols and blocked bisphenols as light stabilizers for yellow image dyes has been known for many years. These materials have been described in detail in, e.g., BP 1,267,287, DE 4,307,439, DE 4,320,828, EP 508,398, EP 538,862, U.S. Pat. No. 4,782,011, U.S. Pat. No. 5,294,530, U.S. Pat. No. 5,426,021, U.S. Pat. No. 5,441,855, U.S. Pat. No. 5,441,861, U.S. Pat. No. 5,466,569, and WO 91/008,515. These documents all teach the use of these light stabilizers with yellow dye forming image couplers in the presence of an oil former, or coupler solvent. U.S. Pat. No. 5,059,515 also teaches that these light stabilizers are preferably dissolved in a high boiling solvent, the amount being preferably 10 to 100%, relative to the coupler. It is also noted in U.S. Pat. No. 5,059,515 that if the stabilizers are liquid, they can themselves act as photographic solvents. However, it is stated that it is required for such embodiment that the stabilizers be so free-flowing that they have the properties of known photographic solvents such as di-n-butyl phthalate or tricresyl phosphate.
It would be advantageous to be able to provide a yellow image forming layer in a photographic element which has no or only very low levels of permanent solvent to enable the advantages discussed above, while including light stabilizers to effectively enable good yellow dye light stability.
It has been unexpectedly discovered that the light stability for yellow image dyes formed in an exposed and processed photographic element may be significantly improved where the yellow dyes are formed in a photographic layer which is substantially free of organic solvents, or which contains only a very low level of organic solvents, in combination with a substituted bisphenol light stabilizer compound which is a solid at room temperature.